Sunday 1 September 2013 02.00 EDT
First published on Sunday 1 September 2013 02.00 EDT

Take your pick. David Cameron's defeat over intervention in Syria is the most abject humiliation for a British prime minister on an issue of war and peace since Lord Aberdeen lost a vote over the conduct of the Crimean war. Or since Lord Palmerston was censured over the Don Pacifico affair. Or since Lord North couldn't carry parliament during the American war of independence. I love a historical precedent, but arguing about which one really applies in this case may seem just a little bit irrelevant to the victims of the Assad regime. Let us just say that nothing quite like this has happened for an exceedingly long time. In fact, the more I delve into what I will argue was a needless defeat for the prime minister, the more I conclude that nothing quite like this has ever happened.

Big moments are prompts for predictions about what it means for the future, most of which sound a bit overblown to me. The self-styled "old warhorse" Paddy Ashdown cries that Britain has suddenly become a shrunken nation – "I have never felt more depressed or ashamed"– and asks whether we are content to become another Sweden. George Osborne proposes a period of "national soul-searching" about what sort of role we want to play in the world. Foreign Office types add their lamentations that a great blow has been struck to the "special relationship" with the US and wince when Washington now refers to France as its "oldest ally". Where they are depressed, others are thrilled. The Conservative rebel Crispin Blunt expresses himself "delighted" that Britain has finally shed her "imperial pretension". Many on the left agree.

About one thing these voices are correct. We are going through a period when the appetite for flexing British military muscle – what remains of it – has shrunk. Earlier successes for liberal interventionism – the restoration of democracy in Sierra Leone, the salvation of Kosovo from Slobodan Milosevic – have been eclipsed by the traumatic experience of Iraq. That has rightly left people sceptical about claims based on intelligence and unwilling to take the word of leaders on trust. Even when the principle of intervention might sound fine and the grounds compelling, there is also an understandable fear that it will be screwed up in practice. As David Cameron said in the parliamentary debate, Iraq has "poisoned the well". As Jack Straw remarked, it has also "raised the bar" that a leader must clear to convince his country that action will have a positive result rather than make a bad situation worse.

Until now, it has been most fashionable to concentrate on the impact that Iraq has had on the Labour party. It is true that, for some Labour MPs, opposing action against Assad seemed to be a way of trying to purge their feelings of guilt about voting for the war against Saddam. Yet the more significant aspect of this episode is what it has illuminated about the right. In the vote on Iraq a decade ago, only a tiny number of Tory MPs opposed the invasion. Compare and contrast with Syria, where much less was at stake. Britain's contribution would have been about six Tomahawk missiles – symbolic, but hardly militarily decisive – not participation in a massive ground invasion in pursuit of regime change. Yet even on a noncommittal motion with the promise of a second vote before the limited action proposed, 30 Tories rebelled, 31 didn't vote and many more were in sympathy with the revolt. This is new. Or, rather, it is not new – it is the revival of an old strand of Tory isolationism, encapsulated by Neville Chamberlain's notorious phrase describing Hitler's threat to Czechoslovakia as being "a quarrel in a far away country between people of whom we know nothing".

Yet I think it is much too big a leap to go on to say that Britain as a whole has decided to pull up a drawbridge on the rest of the world. It is only two years ago that the great bulk of the Conservative party voted to intervene in Libya and did so with the support of the large majority of the Labour party. On that occasion, Ed Miliband delivered his backing with one of the most compelling speeches he has made in this parliament, a far, far better one than the tortuous contribution that he made to the Syria debate. I don't think it reasonable to conclude from this one parliamentary vote that Britain has just made an irreversible decision to wash its hands of any global responsibilities.

It is also fair to observe that it is rather useful for the government to try to swivel the focus in this direction. They are happy with a discussion about the state of the national psyche, with the implication that David Cameron's defeat is somehow the country's fault for lacking the moral fibre to do something about Syria. That is easier for them than confronting the hard question: why did the prime minister make such a series of spectacular miscalculations?

For the reasons discussed above, carrying an argument for intervening in Syria was always going to be very challenging, but that didn't mean it was doomed to be impossible. David Cameron lost this one primarily because he and his senior colleagues messed it up.

I offer you my best estimate about what really happened in the runup to the vote. Last weekend, Barack Obama rang the prime minister while he was still on the beach in Cornwall and the US president asked for British support for punitive strikes against Assad. Mr Cameron signed up and made the commitment pretty much without qualification. It would have been awkward for him to display any hesitation. For months before the chemical attack in Damascus, he had been agitating for action against the Assad regime. So when Barack Obama finally proposes doing something, it would have been embarrassing for Mr Cameron to start hedging his position. He would have feared looking ridiculous in the eyes of the White House.

On Tuesday, through the medium of Twitter, he announced the recall of parliament and then said there would be a vote. In this rush, he made two key assumptions: one, that he could carry enough of his own MPs to win; and another that, even if he couldn't, this wouldn't matter because the Labour party would ultimately fall in behind him. Both assumptions turned out to be fatally wrong. Mr Cameron's people accuse Mr Miliband of constantly upping Labour's demands as the week went on. There is good evidence that he did. But this should not have surprised them. The Labour leader had his own party management problems, something No 10 surely ought to have anticipated. The bad personal chemistry between the two leaders didn't help. In one heated exchange on the Wednesday, Mr Cameron accused the Labour leader of "letting down America" and "taking the side of Lavrov" (the Russian foreign minister). That sort of language was not conducive to crafting a bipartisan position.

On the day, the prime minister made a reasonable fist of advancing the case that the use of chemical weapons is so horrific it demands a response. A poor speech can't be blamed. Yet still he lost, making it all the more humiliating.

Here we come to the strange twists in the tale and why I call it a needless debacle. To anyone watching, it was clear that the government was shedding support as the evening wore on. Tories point the finger at Nick Clegg. He certainly floundered in his closing speech, but it was evident even before he got to his feet that the government was in serious trouble. Some Tory whips claim they had warned Mr Cameron he was heading for disaster several hours before the vote. Could he have averted defeat? Yes. He could have accepted the Labour amendment, an option which was available right up to the moment when the vote was called. After all, several pro-government speakers had already contended that the motions were almost indistinguishable. Embracing the Labour words might have pierced the prime minister's pride, but it would have saved him from defeat and left the door ajar for joining military action once there had been activity at the UN and the weapons inspectors had reported.

Mr Cameron did not take that escape route.

Even when his defeat had been announced, to thunderstruck faces on the government frontbench, it need not have been all over. Ed Miliband got up to ask for a guarantee from the prime minister that he would not use the prerogative powers to join action regardless of the wishes of parliament. Mr Cameron could have simply given that reassurance. It was his choice to declare – snapping "We get that" – that there was now no prospect of Britain's involvement.

Michael Gove became excited. "Disgrace!" he shouted at rebel Tories. "You're a disgrace!" I understand the fury of one of the cabinet's most ardent interventionists. It is natural that he would be angry to see his leader weakened at home and diminished abroad. Yet the true object of the ire of the education secretary and any other frustrated interventionist ought to be David Cameron. This was not just a bad defeat for him. It was an unnecessary one.